Add prerelease compatibility check (#8020)

## Summary

Closes #7977. Makes `PythonDownloadRequest` account for the prerelease
part if allowed. Also stores the prerelease in `PythonInstallationKey`
directly as a `Prerelease` rather than a string.

## Test Plan

Correctly picks the relevant prerelease (rather than picking the most
recent one):
```
λ cargo run python install 3.13.0rc2
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.17s
     Running `target/debug/uv python install 3.13.0rc2`
Searching for Python versions matching: Python 3.13rc2
cpython-3.13.0rc2-macos-aarch64-none ------------------------------ 457.81 KiB/14.73 MiB                                                                                                                    ^C

λ cargo run python install 3.13.0rc3                 
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.17s
     Running `target/debug/uv python install 3.13.0rc3`
Searching for Python versions matching: Python 3.13rc3
Found existing installation for Python 3.13rc3: cpython-3.13.0rc3-macos-aarch64-none
```

crates/uv-python/src/discovery.rs

@@ -1869,7 +1869,13 @@ impl VersionRequest {
         }
     }
 
-    pub(crate) fn matches_major_minor_patch(&self, major: u8, minor: u8, patch: u8) -> bool {
+    pub(crate) fn matches_major_minor_patch_prerelease(
+        &self,
+        major: u8,
+        minor: u8,
+        patch: u8,
+        prerelease: Option<Prerelease>,
+    ) -> bool {
         match self {
             Self::Any | Self::Default => true,
             Self::Major(self_major, _) => *self_major == major,
@@ -1879,14 +1885,14 @@ impl VersionRequest {
             Self::MajorMinorPatch(self_major, self_minor, self_patch, _) => {
                 (*self_major, *self_minor, *self_patch) == (major, minor, patch)
             }
-            Self::Range(specifiers, _) => specifiers.contains(&Version::new([
-                u64::from(major),
-                u64::from(minor),
-                u64::from(patch),
-            ])),
-            Self::MajorMinorPrerelease(self_major, self_minor, _, _) => {
+            Self::Range(specifiers, _) => specifiers.contains(
+                &Version::new([u64::from(major), u64::from(minor), u64::from(patch)])
+                    .with_pre(prerelease),
+            ),
+            Self::MajorMinorPrerelease(self_major, self_minor, self_prerelease, _) => {
                 // Pre-releases of Python versions are always for the zero patch version
                 (*self_major, *self_minor, 0) == (major, minor, patch)
+                    && prerelease.map_or(true, |pre| *self_prerelease == pre)
             }
         }
     }

crates/uv-python/src/downloads.inc.mustache

@@ -3,7 +3,7 @@
 // Generated with `{{generated_with}}`
 // From template at `{{generated_from}}`
 
-use std::borrow::Cow;
+use uv_pep440::{Prerelease, PrereleaseKind};
 
 pub(crate) const PYTHON_DOWNLOADS: &[ManagedPythonDownload] = &[
     {{#versions}}
@@ -12,7 +12,7 @@ pub(crate) const PYTHON_DOWNLOADS: &[ManagedPythonDownload] = &[
             major: {{value.major}},
             minor: {{value.minor}},
             patch: {{value.patch}},
-            prerelease: Cow::Borrowed("{{value.prerelease}}"),
+            prerelease: {{value.prerelease}},
             implementation: LenientImplementationName::Known(ImplementationName::{{value.name}}),
             arch: Arch(target_lexicon::Architecture::{{value.arch}}),
             os: Os(target_lexicon::OperatingSystem::{{value.os}}),

crates/uv-python/src/downloads.rs

@@ -260,8 +260,17 @@ impl PythonDownloadRequest {
                 return false;
             }
         }
+        // If we don't allow pre-releases, don't match a key with a pre-release tag
+        if !self.allows_prereleases() && key.prerelease.is_some() {
+            return false;
+        }
         if let Some(version) = &self.version {
-            if !version.matches_major_minor_patch(key.major, key.minor, key.patch) {
+            if !version.matches_major_minor_patch_prerelease(
+                key.major,
+                key.minor,
+                key.patch,
+                key.prerelease,
+            ) {
                 return false;
             }
             if version.is_freethreaded() {
@@ -269,10 +278,6 @@ impl PythonDownloadRequest {
                 return false;
             }
         }
-        // If we don't allow pre-releases, don't match a key with a pre-release tag
-        if !self.allows_prereleases() && !key.prerelease.is_empty() {
-            return false;
-        }
         true
     }
 

crates/uv-python/src/installation.rs

@@ -1,4 +1,3 @@
-use std::borrow::Cow;
 use std::fmt;
 use std::str::FromStr;
 
@@ -6,7 +5,7 @@ use tracing::{debug, info};
 
 use uv_cache::Cache;
 use uv_client::BaseClientBuilder;
-use uv_pep440::Version;
+use uv_pep440::{Prerelease, Version};
 
 use crate::discovery::{
     find_best_python_installation, find_python_installation, EnvironmentPreference, PythonRequest,
@@ -224,7 +223,7 @@ pub struct PythonInstallationKey {
     pub(crate) major: u8,
     pub(crate) minor: u8,
     pub(crate) patch: u8,
-    pub(crate) prerelease: Cow<'static, str>,
+    pub(crate) prerelease: Option<Prerelease>,
     pub(crate) os: Os,
     pub(crate) arch: Arch,
     pub(crate) libc: Libc,
@@ -236,7 +235,7 @@ impl PythonInstallationKey {
         major: u8,
         minor: u8,
         patch: u8,
-        prerelease: String,
+        prerelease: Option<Prerelease>,
         os: Os,
         arch: Arch,
         libc: Libc,
@@ -246,7 +245,7 @@ impl PythonInstallationKey {
             major,
             minor,
             patch,
-            prerelease: Cow::Owned(prerelease),
+            prerelease,
             os,
             arch,
             libc,
@@ -265,7 +264,7 @@ impl PythonInstallationKey {
             major: version.major(),
             minor: version.minor(),
             patch: version.patch().unwrap_or_default(),
-            prerelease: Cow::Owned(version.pre().map(|pre| pre.to_string()).unwrap_or_default()),
+            prerelease: version.pre(),
             os,
             arch,
             libc,
@@ -279,7 +278,12 @@ impl PythonInstallationKey {
     pub fn version(&self) -> PythonVersion {
         PythonVersion::from_str(&format!(
             "{}.{}.{}{}",
-            self.major, self.minor, self.patch, self.prerelease
+            self.major,
+            self.minor,
+            self.patch,
+            self.prerelease
+                .map(|pre| pre.to_string())
+                .unwrap_or_default()
         ))
         .expect("Python installation keys must have valid Python versions")
     }
@@ -306,7 +310,9 @@ impl fmt::Display for PythonInstallationKey {
             self.major,
             self.minor,
             self.patch,
-            self.prerelease,
+            self.prerelease
+                .map(|pre| pre.to_string())
+                .unwrap_or_default(),
             self.os,
             self.arch,
             self.libc

crates/uv-python/src/interpreter.rs

@@ -157,10 +157,7 @@ impl Interpreter {
             self.python_major(),
             self.python_minor(),
             self.python_patch(),
-            self.python_version()
-                .pre()
-                .map(|pre| pre.to_string())
-                .unwrap_or_default(),
+            self.python_version().pre(),
             self.os(),
             self.arch(),
             self.libc(),

crates/uv-python/template-download-metadata.py

@@ -17,6 +17,7 @@ Usage:
 import argparse
 import json
 import logging
+import re
 import subprocess
 import sys
 from pathlib import Path
@@ -29,6 +30,7 @@ WORKSPACE_ROOT = CRATE_ROOT.parent.parent
 VERSION_METADATA = CRATE_ROOT / "download-metadata.json"
 TEMPLATE = CRATE_ROOT / "src" / "downloads.inc.mustache"
 TARGET = TEMPLATE.with_suffix("")
+PRERELEASE_PATTERN = re.compile(r"(a|b|rc)(\d+)")
 
 
 def prepare_name(name: str) -> str:
@@ -61,11 +63,21 @@ def prepare_arch(arch: str) -> str:
             return arch.capitalize()
 
 
+def prepare_prerelease(prerelease: str) -> str:
+    if not prerelease:
+        return "None"
+    if not (match := PRERELEASE_PATTERN.match(prerelease)):
+        raise ValueError(f"Invalid prerelease: {prerelease!r}")
+    kind, number = match.groups()
+    return f"Some(Prerelease {{ kind: PrereleaseKind::{kind.capitalize()}, number: {number} }})"
+
+
 def prepare_value(value: dict) -> dict:
     value["os"] = value["os"].title()
     value["arch"] = prepare_arch(value["arch"])
     value["name"] = prepare_name(value["name"])
     value["libc"] = prepare_libc(value["libc"])
+    value["prerelease"] = prepare_prerelease(value["prerelease"])
     return value